This invention relates to the supporting of semiconductor wafers for processing in a vacuum and particularly to the supporting of wafers, particularly relatively fragile wafers, for the transfer of heat to and from the wafers by gas conduction.
Semiconductor wafers and other substrates that are supported in vacuum chambers for processing by processes such as reactive ion-etching (RIE), plasma etching, ion-beam etching, etching, ion-beam deposition, physical vapor deposition (PVD), chemical vapor deposition (CVD), and other processes. Applications using this type of equipment for example, are those set forth in Thin Film Processes (John L. Vossen and Werner Kern, eds), Academic Press Inc., Orlando, Fla., 1978; and Thin Film Processes II (John L. Vossen and Werner Kern, eds), Academic Press Inc., San Diego, Calif., 1991. In such applications, the substrates must usually be supported in a way that will facilitate the transfer of heat to or from the substrate. Usually it is also desirable to transfer the heat uniformly across the extent of the wafer.
It has been recognized that one effective way to heat and cool substrates in a vacuum chamber is by a process known as backside gas conduction. Backside gas conduction is a process by which heat is transferred between the wafer and a heat transfer body of a wafer supporting chuck by conduction of gas atoms or molecules between the body of the chuck and the wafer. Backside gas conduction takes place in a gas when the molecules of the gas, which leave one surface with heat energy acquired from that surface, arrive at the other surface without colliding with other molecules or particles when traversing the intervening space, thereby delivering energy to that other surface most efficiently. Most of the molecules of gas leaving one surface strike the other surface without in-flight collisions when the spacing of the surfaces is, by definition, less than the mean free path of particles in the gas. Backside gas conduction has been discussed in a number of U.S. patents, for example: in U.S. Pat. Nos. 4,457,359; 4,508,161; 4,512,391; 4,542,754; 4,680,061, 4,743,570; 4,903,754; 4,909,314 and 4,949,783. Other mechanisms of conducting heat to and from a substrate are also used, including gas convection and radiation.
Much of the focus of prior art efforts to provide heat transfer to and from a substrate has been on the manner of holding the substrate to the chuck in a way that holds the substrate to the chuck so that the most effective heat transfer occurs. As exemplified by the efforts used for backside gas conduction, holding the substrate close enough to the chuck to insure that heat transfer by gas conduction occurs has been the subject of much attention. Supporting the substrate for effective heat transfer has led to clamping solutions that have subjected the substrates to certain forces, which, when the substrate is a semiconductor wafer of relatively fragile characteristics, such as gallium arsenide wafers (GaAs) for example, can break or otherwise damage the wafer.
Further, gas leakage between the gap behind the wafer and the gas in the processing chamber can adversely affect the process, either by adversely, affecting the chemistry involved in the process or by entering the film on the substrate and degrading the product being formed on the substrate. In addition, where aggressive process gases or other harmful gases are involved in the process, these gases can contact the back of the wafer or components of the chuck, where reactions, deposits, electrical breakdowns or other damage to the wafer can occur. This has motivated the use of seals in forced contact with the wafer, and has resulted in further risk of damaging the wafers, particularly if they are of a fragile type.
Accordingly, better ways are needed for clamping semiconductor wafers with respect to a heat transfer surface to facilitate heat transfer between the wafer and the heat transfer surface in a vacuum processing chamber.
An objective of the present invention is to provide for the heating or cooling of semiconductor wafers and other substrates in a vacuum processing chamber, and particularly, to support such wafers, especially gallium arsenide and other fragile wafers, for heating or cooling in a vacuum. One particular objective of the invention is to support semiconductor wafers and other substrates in a vacuum for heating by backside gas conduction.
According to principles of the present invention, a substrate holder is provided with resilient structure to minimize localized forces on a wafer while being handled and supported in a vacuum processing chamber. According to other principles of the invention, the substrate holder is adapted to support the wafer at a predetermined distance from a heat transfer body of the substrate holder so as to facilitate heat transfer between the body and the wafer. The substrate holder so provided is particularly suited for exchange of heat with the wafer by backside gas conduction.
According to certain embodiments of the invention, resilient structure is provided: for engaging and moving a wafer between a wafer exchange position accessible to a transfer mechanism and a wafer processing position, and for forming a gas tight cavity that includes a narrow gap between the wafer and the heat transfer body that includes forming seals around the wafer and the heat transfer body.
In certain embodiments of the invention, a substrate support is provided in a vacuum processing chamber having a heat transfer body, a substrate seat member configured to resiliently support the semiconductor wafer by contacting the backside of the wafer around its edge, and a clamp ring that clamps the wafer to the substrate seat member, which may also be through resilient contact.
In an illustrated embodiment of the invention, the seat member is vertically moveable on guides attached to the heat transfer body and is biased away from the body by compression springs. The seat member normally rests away from the body in a wafer exchange position and moves, by force exerted against the front side of the wafer by the clamp ring, to a processing position spaced by a narrow gap from the heat transfer body.
Also in the illustrated embodiment, the clamp ring is vertically moveably supported on the chamber wall and is moved in guides by an actuator. The clamp ring stands away from the body and seat member to allow exchange of the wafer on the seat member and moves toward the body and contacts the wafer around its edge to force the wafer and seat member on which the wafer is supported downward against the force of the springs to the processing position. The clamp ring is supported and configured, relative to the heat transfer body, so as to precisely establish and maintain a desired thickness of the gap during processing. Such clamp rings support and configuration may be adjustable to accommodate different gap thicknesses.
According to certain embodiments of the invention, a resilient annular seal is positioned on the inner edge of the clamp ring to contact the wafer around its edge and form a gas tight cavity that includes the gap between the heat transfer body and the wafer when the seat member is forced to its lower processing position by the clamp ring.
Also according to the illustrated embodiments of the invention, the heat transfer body is provided with an upwardly facing heat transfer surface approximately corresponding in size to that of a semiconductor wafer to be processed, and may also be provided with an upwardly facing annular outer sealing surface that surrounds the heat transfer surface. The substrate seat member is configured to support an upwardly facing semiconductor wafer and move vertically between i) an upper wafer exchange position under the force of compression springs and ii) a lower processing position spaced by the narrow gap from the heat transfer surface under force of the clamp ring acting against the force of the compression springs. The clamp ring may have the annular seal on the wafer side of its inner edge adapted to contact and form a gas tight seal against the frontside of the wafer. In addition, an annular outer seal may be provided between the outer sealing surface of the heat transfer body and the clamp ring that forms an annular channel within the cavity in communication with the gap when the seat member is in its lower processing position.
The substrate support may be incorporated into semiconductor wafer vacuum processing machines that have control systems that control the pressure of gas within the gap such that the mean free path of particles of the gas is greater than the thickness of the gap, thereby efficiently facilitating transfer of heat between the heat transfer body and the surface of the wafer by gas conduction.
The invention allows for gentle handling of fragile wafers, for example, gallium arsenide wafers that are particularly brittle. Contact of the wafer by the clamp ring through a resilient seal and support of the seat plate on compression springs contribute to this capability. Gas from the chamber is isolated from the backside of the wafer, avoiding harmful deposits on the backside of the wafer or substrate support surface. Further, when a seal is made with the frontside of the wafer, higher pressure in the gap than in the chamber results in force on the wafer that presses the wafer against the seal.
These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description, in which: